Mexican authorities found five abandoned bullet-riddled and bloodstained vehicles on Wednesday, fueling their hunt for drug gang killers following a wave of border-region slayings and clashes with soldiers that left 21 people dead, an official said.The hours-long skirmishes around the town of Villa Ahumada on Tuesday were part of a wave of drug violence that has engulfed parts of Mexico _ and has even spilled across the border _ as the army confronts savage narcotics cartels that are flush with drug money and guns from the U.S.President Felipe Calderon says more than 6,000 people died last year in drug-related violence, and U.S. authorities have reported a spike in killings, kidnappings and home invasions linked to the cartels _ some of it in cities far from the border, such as Phoenix and Atlanta.

Investigators on Wednesday were searching for assailants after finding five abandoned vehicles near Villa Ahumada, where gunmen a day earlier had kidnapped nine people, starting the violence.

They executed six of the kidnap victims along the PanAmerican Highway outside of the town, said Enrique Torres, spokesman for a joint military-police operation in Chihuahua state.Villa Ahumada is 80 miles (130 kilometers) south of the border city of El Paso, Texas.An army convoy heading toward Villa Ahumada to investigate reports of the kidnappings Tuesday came across gunmen who had just executed the six kidnap victims, Torres said.A shootout between gunmen and soldiers ensued in which seven gunmen and one soldier died, Torres said. Another soldier was wounded.Soldiers rescued the three remaining kidnap victims and took them into custody for questioning, Torres said. The men say they are businessmen and were wrongly accused by their captors of belonging to the Sinaloa cartel.In the meantime, other gunmen fled on foot as soldiers rappelled down from military helicopters to chase them through the snow-covered desert.Further down the highway, a series of other shootouts left seven more assailants dead.Villa Ahumada, a town of 1,500 people, was virtually taken over by drug gangs last year when attackers killed two consecutive police chiefs and two officers. The rest of the 20-member force resigned in fear, forcing the Mexican military to take over for months.Unable to hire new recruits, the town hired unarmed residents to keep watch and alert state police about crime.The army was patrolling the town's streets Wednesday.Also Wednesday in the town of Reforma in Chiapas state, three suspected drug hitmen died during a shootout with police. Assailants opened fire after police raided a safe house for arms and drugs, a Reforma municipal police spokesman said. He was not authorized to give his name.